Int'l Publication No. WO 2006/011950 discloses a conventional ignition plug. As shown in FIGS. 14 and 15 of the present application, a conventional ignition plug includes an electrically conductive metallic shell 101 having a through hole 100 extending therethrough in the axial direction. An insulator 102 is attached to the through hole 100 of the metallic shell 101, and a center electrode 103 attached to the insulator 102. Defining the side where the center electrode 103 is disposed as a “front end side,” the metallic shell 101 has an opening (front end opening) 104 on the front end side. The ignition plug includes a cap member 107 having a hole 106. Cap member 107 is provided at the front end of the metallic shell 101 and covers the front end opening 104 of the metallic shell 101, to thereby form an ignition chamber 105. Four semi-circular ground electrodes 108 project from the wall surface of the ignition chamber 105 toward the circumferential surface of the center electrode 103.
Such an ignition plug, having the ignition chamber 105 at the front end of the metallic shell 101 (hereinafter also referred to as a “prechamber plug”), introduces an air-fuel mixture within a combustion chamber of an internal combustion engine into the ignition chamber 105 via the hole 106 of the cap member 107, produces spark discharge at a gap G between the center electrode 103 and the ground electrode 108 so as to ignite the mixture, to thereby generate a flame in the ignition chamber 105. The flame is jetted from the hole 106 of the cap member 107 into the combustion chamber of the internal combustion engine, and is spread across the entire combustion chamber. As described above, such a prechamber plug is excellent in ignition performance, and allows for construction of an internal combustion engine which is high in combustion speed. Therefore, such a prechamber plug is used mainly for internal combustion engines, such as engines for cogeneration and gas engines for compressors.
Since the ignition plug ignites an air-fuel mixture by producing spark discharge at the gap G between the center electrode 103 and the ground electrode 108, whether or not the size of the gap G falls within a prescribed range is an important factor which determines its ignition performance.
However, in the prechamber plug, since the center electrode 103 and the ground electrodes 108 are located within the ignition chamber 105, correction of the gap G (gap adjustment) is structurally difficult to perform. Therefore, the conventional prechamber plug is designed such that the size of the gap G is brought into a prescribed range through accurate assembly of the metallic shell 101, the insulator 102, and the center electrode 103 during a manufacturing process.
However, by means of manufacturing 25 conventional prechamber plugs (the number of ground electrodes=4) on a trial basis and measuring 100 gaps G in total, the present inventor found that, despite the target range for the gap G being set to 0.27 mm to 0.33 mm, in actuality, the size of the gap G greatly varied within a range of 0.14 mm to 0.46 mm, as indicated by solid lines in the graph of FIG. 9.
The present invention has been accomplished in view of the above-described problem, and its object is to provide a prechamber plug whose spark discharge gap is readily corrected (gap adjustment is readily performed), and a method of manufacturing the prechamber plug.